Process for manufacturing toothed sprockets for rewinding devices of safety belts

ABSTRACT

A process for manufacturing toothed sprockets, comprising the steps of pressure die-casting a molten metal alloy into a runner ( 7 ) communicating with a molding cavity ( 6 ) so as to define at least one toothed sprocket ( 1 ) associated with a respective die-casting sprue; the die-casting sprue is cut off leaving a residual sprue portion ( 8 ) projecting from the sprocket ( 1 ). Subsequently the toothed sprocket ( 1 ) is turned for removing the residual sprue portion ( 8 ) projecting from the sprocket ( 1 ) itself.

FIELD AND BACKGROUND OF THE INVENTION

The present invention pertains to a process for manufacturing toothed sprockets for rewinding devices of safety belts.

It is known that rewinding devices for safety belts, of the type commonly used on motor-vehicles for example, usually comprise a toothed sprocket coaxially associated with a roller for winding up the safety belt. Rotation of the sprocket is locked when, upon intervention of an accelerometer operating device, a toothed fork is brought to a working condition of mutual meshing with the sprocket toothing.

Since the toothed sprocket is a component on which persons' safety relies in case of accident, strict parameters must be observed in terms of geometric and dimensional tolerance, to supply suitable reliability guarantees in time.

These requirements are presently met by making the toothed sprockets of an aluminum alloy molded by pressure die-casting.

In detail, die-casting is carried out through injection of a molten aluminum alloy into a mold in which a plurality of molding cavities, or so-called “mold impressions” are defined, each corresponding to a toothed sprocket to be obtained, mutually connected by runners opening into at least one aperture or gate for admission of molten metal.

These runners communicate with the molding cavity according to a configuration of the so-called “comb-like” type, to subtend an angular sector of a peripheral circumferential region of the cavity itself adapted to define the toothing of the respective sprocket. More particularly, each runner branches out in the vicinity of the respective molding cavity, so as to define a plurality of branches each converging at a respective tooth of the toothing.

Then from the die-casting process, following solidification of the metal alloy inside the mold, a plurality of toothed sprockets are obtained which are linked at an arched portion of the toothing, to the respective branches of a molding sprue defined by the material solidified inside the connecting runners. The casting drawn out of the die-casting mold is submitted to a cutting step to remove the molding sprue from each toothed sprocket.

This cutting step causes formation of one or more residual sprue portions, commonly referred to as “flashes”, projecting from the individual sprocket teeth.

Flashes must be eliminated by means of accurate and complicated surface finishing operations that, if not executed with due care, can involve many machine rejections during the subsequent steps for quality control or, in the absence of an accurate control, can give rise to many drawbacks mainly connected with malfunctions of the sprocket during use.

As mentioned above, in fact the sprocket, and in particular the sprocket toothing, must observe severe parameters in terms of geometric and dimensional tolerance, to provide suitable reliability guarantees.

The Applicant has further perceived that admission of the metal alloy into the molding cavity according to the above described comb-like configuration gives rise to metallurgic features that are not fully satisfactory and can be in any case improved.

OBJECT OF THE INVENTION

In the light of the above, the present invention aims at providing a process for manufacturing toothed sprockets capable of overcoming the problems found in the known art.

It is in particular an object of the invention to propose a process for manufacturing toothed sprockets capable of eliminating each residual sprue portion from the sprocket, without interfering with the geometrical and dimensional tolerance parameters of the sprocket itself.

SUMMARY OF THE INVENTION

The technical task mentioned and the objects specified are substantially achieved by a process for manufacturing toothed sprockets for rewinding devices of safety belts, comprising the steps of: pressure die-casting a molten metal alloy in a communication runner opening into a molding cavity, for defining a toothed sprocket associated with a respective die-casting sprue; removing the die-casting sprue from the toothed sprocket; lathe-shaping or turning the toothed sprocket for removing the residual sprue portions projecting from the sprocket.

BRIEF DESCRIPTION OF THE DRAWINGS

Description of a preferred but not exclusive embodiment of a process for manufacturing toothed sprockets for rewinding devices of safety belts is now given by way of non-limiting example and illustrated in the accompanying drawings, in which:

FIG. 1 is a perspective view of a toothed sprocket during the manufacturing process of same in accordance with the present invention;

FIGS. 2 a and 2 b show a plan view and an elevation side view respectively of the toothed sprocket seen in FIG. 1;

FIGS. 3 a and 3 b show a plan view and an elevation side view respectively of the toothed sprocket during a turning step provided in the process in reference;

FIG. 4 is a perspective view of the sprocket in FIG. 1 at the end of the manufacturing process according to the invention; and

FIG. 5 is a section view of a mold for manufacturing the toothed sprocket illustrated in FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

With reference to the drawings, a toothed sprocket obtainable by a process in accordance with the present invention has been generally denoted by reference numeral 1.

The toothed sprocket 1 is designed to be used for locking a winding roller in a rolling-up device for safety belts, in particular for use on cars, not shown as it can be made in any known manner.

Sprocket 1 has a substantially cylindrical conformation, of circular cross-section, and defines a tubular body 2 extending along a respective longitudinal axis “X”. A circumferential toothing extends around the tubular body 2; it is coaxial with said longitudinal axis “X” and is adapted to mesh with an engagement fork, in a locked condition of said winding roller.

Sprocket 1 further has a circular portion 4 coaxial with and disposed close to toothing 3 and having a diametrical bulkiness smaller than toothing 3.

The process of the invention contemplates die-casting of an aluminum alloy or other suitable metal alloy into a mold 5 (only partly shown in FIG. 5) having one or more molding cavities 6 the shape of which matches that of corresponding sprockets 1 that will be formed after said die-casting step. Cavities 6 of mold 5 communicate with each other through runners 7 (FIG. 5) opening into at least one opening or gate for entry of the metal alloy. During the die-casting step, the molten metal alloy reaches each molding cavity 6 through the respective runner 7 at which, following cooling, a die-casting sprue is formed with which sprocket 1 remains in engagement.

Note that runner 7 communicates with said cavity 6 at a region close to the circumferential toothing 3 of sprocket 1. More particularly, runner 7 opens into a portion of cavity 6 extending like a continuous circular sector at said circular portion 4, axially offset relative to the circumferential toothing 3 of sprocket 1.

Under this situation, the metal alloy is caused to flow through runner 7 externally of toothing 3 so as to form, as a result of cooling, a sprue coupled to sprocket 1 at a portion of same designed to define the circular portion 4.

After the die-casting step, sprocket 1 is drawn out of the cavity to be submitted to a step of removing the die-casting sprue, by cutting for example.

When removal has been completed, a residual sprue portion 8 is still anchored to sprocket 1, at a position axially close to toothing 3. As shown in FIGS. 1, 2 a and 2 b, said residual sprue portion 8 is substantially in the form of an arc of a circle and radially projects beyond the diametrical dimensions of the whole sprocket 1.

In fact cutting is preferably executed out of the diametrical dimensions of the circumferential toothing 3, so as to avoid any possible damage to the toothing 3 itself.

Sprocket 1 provided with the residual sprue portion 8 is subsequently turned in order to remove this residual excess portion 8. The turning step (FIGS. 3 a and 3 b) is carried out by rotating sprocket 1 around the respective longitudinal axis “X” and radially moving a cutter 9 or other suitable cutting-off device close to the sprocket itself so as to eliminate said residual projecting sprue portion 8 and define the circular portion 4.

Advantageously, the turning step reduces the diametrical size of the toothed sprocket 1 at the circular portion 4 and, if necessary, can define a shoulder 4 a axially spaced apart from toothing 3, on the circular portion 4 itself.

The invention achieves important advantages.

First of all, the above described process allows the residual sprue portions to be eliminated in a very simple, quick and efficient manner, with important advantages in terms of reduction of the machine rejections and of the process costs in general.

In fact, being runner 7 disposed externally of the axial dimensions of toothing 3, elimination of the residual sprue portions 8 can be carried out without any risk of the toothing profile being impaired, since the geometric and dimensional accuracy of said profile is fundamental for correct operation of the locking system in safety belts.

In addition, due to the shape of runner 7 entering the cavity 6 as a continuous circular sector, quicker and more even filling of the cavity itself can be achieved and therefore better metallurgic qualities are given to the toothed sprockets obtained in accordance with the present invention.

The turning step, in addition to eliminating the residual sprue portion 8, can be also exploited so as to give the circular portion 4 and/or the possible shoulder 4 a any diametrical and/or axial size, thus achieving a greater process versatility in relation to the different types of locking devices for which the toothed sprocket 1 is to be used. 

1. A process for manufacturing toothed sprockets for rewinding devices of safety belts, comprising the steps of: pressure die-casting a molten metal alloy in a communication runner opening into a molding cavity, so as to define a toothed sprocket associated with a respective die-casting sprue; removing the die-casting sprue from the toothed sprocket; lathe-shaping or turning the toothed sprocket so as to remove a residual sprue portion projecting from the sprocket itself.
 2. The process as claimed in claim 1, wherein said sprocket has a substantially cylindrical conformation of circular cross-section, extending around a respective longitudinal axis, and defining a circumferential toothing coaxial with said longitudinal axis.
 3. The process as claimed in claim 1, wherein in the die-casting step said metal alloy flows into the molding cavity close to the circumferential toothing of the sprocket.
 4. The process as claimed in claim 3, wherein in the die-casting step said metal alloy flows into the molding cavity externally of the axial dimensions of said circumferential toothing.
 5. The process as claimed in claim 3, wherein in the die-casting step said metal alloy flows into a portion of the molding cavity extending in a continuous circular sector.
 6. The process as claimed in claim 3, wherein in the die-casting step said metal alloy flows into the molding cavity at a circular portion coaxial with said toothing and axially offset relative to the same.
 7. The process as claimed in claim 2, wherein the residual sprue portion radially projects from a circular portion of the sprocket that is axially offset relative to the toothing.
 8. The process as claimed in claim 1, wherein the residual sprue portion radially projects in the form of a continuous arc of a circle from a circular portion of the sprocket.
 9. The process as claimed in claim 2, wherein following the turning step, a shoulder axially spaced apart from the toothing is defined.
 10. The process as claimed in claim 1, wherein said die-casting step is carried out in a plurality of runners associated with each other and each of which is in communication with a respective molding cavity.
 11. The process as claimed in claim 2, wherein removal of the sprue is carried out through cutting out of the diametrical dimension of the circumferential toothing. 